The present invention relates to a method of polishing a substrate whereby chemical mechanical polishing (CMP) is performed with respect to a semiconductor substrate of silicon or a liquid-crystal substrate to flatten a surface thereof, to an apparatus for polishing a substrate, and to an apparatus for holding a substrate to be polished by the above method of polishing a substrate.
From the 1990s, as semiconductor and liquid-crystal substrates processed by CMP technology have had larger diameters on the order of 10 cm or more, there have been increasing tendencies toward single-wafer polishing. In the case of polishing a semiconductor substrate, in particular, equal polishing should be performed with respect to the entire surface thereof since a design rule of 0.5 .mu.m or less has been used to define extremely small lines and spaces on the semiconductor substrate.
Below, a method and apparatus for polishing a substrate according to a first conventional embodiment will be described with reference to the drawings.
FIG. 13 schematically shows the structure of the apparatus for polishing a substrate according to the first conventional embodiment, in which is shown a table 51 including: a pad table 51a having a flat surface which is made of a rigid material; a rotary shaft 51b extending downwardly from the back face of the pad table 51a; and rotating means (not shown) for rotating the rotary shaft 51b. To the top surface of the pad table 51a of the table 51 is adhered an elastic polishing pad 52. Above the polishing pad 52 is provided a substrate holding head 54 which holds and rotates a substrate 53. The substrate 53 is pressed against the polishing pad 52 while being rotated by the substrate holding head 54. An abrasive agent 55 in a prescribed amount is supplied dropwise from an abrasive supply pipe 56 onto the polishing pad 52.
In the apparatus for polishing a substrate thus constituted, the polishing pad 52 supplied with the abrasive agent 55 is rotated by rotating the table 51 and the substrate 53 held by the substrate holding head 54 is pressed against the rotating polishing pad 52 so that a surface of the substrate 53 is polished under pressure and at a relative speed.
In this process, if the surface of the substrate 53 being polished is rugged, projecting portions thereof are removed at an increased relative polishing rate since their contact pressure with the polishing pad 52 is high. On the other hand, recessed portions thereof are barely polished since their contact pressure with the polishing pad 52 is low. Consequently, the surface of the substrate 53 to be polished becomes less rugged and more smooth.
However, the first conventional embodiment mentioned above presents the following problems, which will be described below with reference to FIG. 14 and FIGS. 15(a) to 15(c).
FIG. 14 is a schematic view illustrating a polishing method implemented by the above first conventional embodiment. FIGS. 15(a) to 15(c) are schematic views elucidating the problems of the first conventional embodiment. By way of example, a description will be given to a problem in polishing an oxide film 57 having a rugged surface which is formed on the substrate 53 of silicon.
As described above, when the substrate 53 held by the substrate holding head 54 is pressed against the polishing pad 52 as shown in FIG. 14, the projecting portions of the oxide film 57 are removed since their contact pressure with the polishing pad 52 is high, while the recessed portions thereof are barely polished since their contact pressure with the polishing pad 52 is low, resulting in a flat surface of the oxide film 57 with a reduced degree of ruggedness.
In this process, the substrate 53 held by the substrate holding head 54 is pressed against the polishing pad 52 with an equal force. However, if the pad table 51a of the table 51 has a warped surface and the polishing pad 52 is elastically deformed as shown in the left part of FIG. 15(a) or if the thickness of the substrate 53 has variations as shown in the left part of FIG. 15(b), the contact pressure between the oxide film 57 and the polishing pad 52 is not equal but differs from one portion to another even when the substrate 53 is pressed against the polishing pad 52 with an equal force. As a result, the polishing rate is higher at portions under higher contact pressure (these portions of the oxide film 57 which are in contact with the projecting portions of the pad table 51a or polishing pad 52 and which correspond to thicker portions of the substrate 53) , while the polishing rate is lower at portions under lower contact pressure (these portions of the oxide film 57 which are in contact with the recessed portions of the pad table 51a or polishing pad 52 and which correspond to thinner portions of the substrate 53), resulting in an unequal amount of polishing with respect to the oxide film 57.
To overcome the problem, it is possible to lessen variations in contact pressure resulting from the ruggedness of the pad table 51a or polishing pad 52 or from varied thickness of the substrate 53 by increasing the ease with which the polishing pad 52 is elastically deformed in accordance with the ruggedness of the pad table 51a or polishing pad 52 or with varied thickness of the substrate 53, thereby achieving an equal contact pressure. However, when the polishing pad 52 is softened, it is deformed in accordance with the rugged configuration of the oxide film 57, so that even the recessed portions of the oxide film 57 are brought into contact with the polishing pad 52 and undergo polishing. Accordingly, variations in level on the surface of the oxide film 47 cannot be lessened as shown in the right part of FIG. 15(c).
To overcome the problem, there has been proposed a technique which utilizes the deformation of the substrate 63 against the elastic deformation of the polishing pad 62.
FIG. 16 shows a method and apparatus for polishing a substrate according to a second conventional embodiment, wherein an elastic polishing pad 62 is adhered to the top surface of a table 61. The bottom portion of a substrate holding head 64 for holding a substrate 63 is formed with a recessed portion 65. The substrate 63 is solidly supported by a plate-like elastic member 66 which can be elastically deformed in the recessed portion 65 of the substrate 63. The substrate holding head 64, elastic member 66, and substrate 63 define a hermetically sealed space 67 into which a gas under controlled pressure is introduced through a gas supply path 68. The gas under pressure introduced into the hermetically sealed space 67 presses the substrate 63 solidly supported by the elastic member 66 against the polishing pad 62, so that the pressure on the upper face of the substrate 63 achieves equal polishing.
However, since the second conventional embodiment is so constituted that the substrate 63 is solidly supported by the plate-like elastic member 66, there arises a first problem of a complicated mechanism and intricate operation of mounting and dismounting the substrate 63.
In the foregoing first conventional embodiment, on the other hand, the polishing pad 52 have different thicknesses from one portion to another due to elastic deformation caused by pressure during polishing or loading is caused by abrasive grains which have been jammed into the surface of the polishing pad 52, so that the surface state of the polishing pad 52 changes and the polishing rate varies. Moreover, a friction between the substrate 53 and the polishing pad 52 causes partial abrasion of the polishing pad 52 or the polishing pad 52 itself is thinned since the polishing pad 52 elastically deformed under pressure during polishing is not restored.
Consequently, the surface of the polishing pad 52 becomes rugged due to varied thickness of the polishing pad 52 so that variations in pressure are produced over the surface of the substrate 53 even if the substrate 53 is pressed against the polishing pad 52 with an equal force, resulting in an unequal amount of polishing with respect to the surface of the substrate.
To avoid an unequal amount of polishing due to a change (fatigue) in the surface state of the polishing pad 52, a dressing process is performed with respect to the polishing pad 52, in which projecting portions are removed from the surface of the polishing pad 52 or the loading is eliminated to maintain the polishing pad 52 in a constant state and thereby accomplish equal polishing with respect to the surface of the substrate 53.
However, the method in which the dressing process is performed with respect to the polishing pad has the following problems.
First, to impart constant flatness to the surface of the polishing pad which undergoes changes caused by abrasion or the like during polishing, it is necessary to control the amount of removal from the polishing pad based on the height and depth of projecting and recessed portions formed during polishing. However, it is extremely difficult to measure the height and depth of the projecting and recessed portions and control the amount of removal since the height and depth of the projecting and recessed portion from the surface level of the polishing pad is as small as several micrometers to several tens of micrometers. In particular, it is extremely difficult to polish the surface of the substrate with such an accuracy as is required in the latest device manufacturing process.
Second, since the removal from the polishing pad by the dressing process reduces the thickness of the polishing pad, the elasticity of the polishing pad changes and the ability to absorb variations in pressure on the surface of the substrate differs from one dressing process to another, so that steady polishing characteristics (surface uniformity and the ability to tolerate variations in level) cannot be obtained.
Third, although it is required to replace polishing pads which are sufficiently thick to implement the polishing characteristics, the replacing operation should be performed manually.
Fourth, although the above first problem can be overcome to a certain extent if variations in pressure on the surface of the substrate is absorbed by the use of a polishing pad which is liable to elastic deformation, the ability to tolerate variations in the level of the substrate is lowered by the use of the polishing pad which is liable to elastic deformation.
Fifth, although the thickness of the polishing pad adhered to the table is measured mechanically, it is difficult to measure the thickness of the polishing pad soaked with an abrasive agent or cleaning water. Moreover, in the case where a sequence of polishing processes are performed on end, it is impossible to measure the thickness of the polishing pad. Furthermore, when measured mechanically, the thickness of the polishing pad can only be estimated empirically based on the amount of polishing and on the amount of abrasion of the polishing pad for such reasons that the distribution of thickness over the surface of the polishing pad cannot be estimated unless measurements are performed at a large number of points and that high-accuracy measurements cannot be performed since the measurement accuracy in a microgage normally used is on the order of 10 .mu.m, though the surface of the polishing pad is formed with projecting and recessed portions which are about several micrometers in height and depth.